1. Field of the Invention
The present invention relates to a semiconductor device operated in a microwave band and, more particularly, to a semiconductor device formed by a Schottky barrier field effect transistor (FET).
2. Description of the Related Art
FIG. 1 shows an example of a conventionally used FET of this type.
A gate electrode 2 of an FET formed on a semiconductor substrate 1 has a very wide gate width so that the FET is appropriately operated in a microwave band. Two input signal supplying pads 2A for receiving a common input signal are arranged at the gate electrode 2. These are arranged in a -shape as a whole. As described above, a plurality of pads (two here) are arranged since it is effective to decrease noise by reducing a gate resistance. For example, an FET having a gate width of 300 .mu.m is used as a so-called X-band or KU-band FET. Note that reference numeral 3 denotes a source electrode wiring layer, and reference numeral 4 denotes a drain electrode wiring layer.
In the conventional microwave FET, the productive yield is decreased by a fluctuation in threshold voltage, resulting in inconvenience. Generally, the threshold voltage of the FET is determined by a thickness of an active layer or a carrier density. Manufacturing fluctuations of them cannot be inevitably prevented. Particularly, the FET for microwave has larger fluctuations because a microwave FET has a recess structure formed by etching a portion under the gate electrode. Accordingly, its threshold voltage often falls outside the range of design values.
If the threshold voltage can be controlled after a process is finished, this problem can be solved. In a normal FET having a narrow gate width, a side gate formed by an ohmic electrode is arranged on a semiconductor substrate near an active gate layer at a substrate portion extended in a gate width direction, and a threshold value can be controlled by a voltage applied to the side-gate electrode formed by the ohmic metal. As described above, in the microwave FET having a wide gate width, however, this method is not effective.